Pfizer and partner BioNTech said Monday that their vaccine against Covid-19 was strongly effective, exceeding expectations with results that are likely to be met with cautious excitement — and relief  in the face of the global pandemic.

The vaccine is the first to be tested in the United States to generate late-stage data. The companies said an early analysis of the results showed that individuals who received two injections of the vaccine three weeks apart experienced more than 90% fewer cases of symptomatic Covid-19 than those who received a placebo. For months, researchers have cautioned that a vaccine that might only be 60% or 70% effective.

Vaccines normally require years of testing and additional time to produce at scale, but scientists are hoping to develop a coronavirus vaccine within 12 to 18 months. Vaccines mimic the virus – or part of the virus – they protect against, stimulating the immune system to develop antibodies. They must follow higher safety standards than other drugs because they are given to millions of healthy people.

So when will the vaccine become available?

Pfizer believes it will be able to supply 50 million doses by the end of this year, and around 1.3 billion by the end of 2021. The UK should get 10 million doses by the end of 2020, with a further 30 million doses already ordered. Exactly who will be immunised first will depend on where Covid is spreading when the vaccine becomes available and in which groups it is most effective.

The UK has not decided, for example, how to prioritise health and care workers who work with the most vulnerable people, relative to those most at risk if they catch the disease. In broad terms, age is, by far, the biggest risk factor for Covid, so the older you are, the sooner you are likely to be vaccinated.

Most experts think the vaccine will not be widely available until the middle of 2021.

Why do we need a vaccine?

If you want your life to get back to normal, then we need a vaccine. Even now, the vast majority of people are still vulnerable to a coronavirus infection. It is only the restrictions on our lives that are preventing more people from dying. But vaccines safely teach our bodies to fight the infection. This can either stop us catching coronavirus in the first place or at least make Covid less deadly.

The vaccine, alongside better treatments, is “the” exit strategy.